As is well known, lithographic color printing is usually performed as a four color separation process. The particular scene or copy to be printed is photographed four separate times, once for black and then once each through filters which will pass only light of one of the three process colors (magenta, cyan, and yellow). Each of the negatives is separately developed and used to create separate printing plates. The four printing plates so created are then used sequentially with the appropriate color process inks to create a properly colored printed image by successive overlays of the transparent inks.
Once the four color separation negatives have been made by photographing the scene or copy, but before they are used to create the printing plates, it is normal practice to create proofs from which the printer, artist, stripper or others involved in the printing process can judge the accuracy of the color renditions. Normally this is done by using each negative to create a corresponding colored transparency. The four transparencies thus created are then overlaid and viewed against a lighted background. The artist, printer, stripper or other viewer can thus see what the finished printed subject will look like. If the proof shows that the color renditions are not what was desired, appropriate adjustments can be made on the negatives or the scene can be rephotographed to correct the colors. Any one or more of the four color separation negatives can be redone in this manner to alter the color balance. These proofing steps can, of course, be repeated until the color rendition is judged to be satisfactory. The separation negatives at that point are then used for producing the printing plates from which the final color print is made.
It is also common in lithographic printing to have layouts which require blank spaces in the printing. Commonly this occurs when the layout calls for a slogan, word, logo or the like to be printed against a colorless background. In the proofing stage this requires the use of a film with an opaque coating such that when the film is overlaid with the four color film transparencies no light passes through the opaque portions and the areas that will be clear of color in the final proof can be observed. Development of the opaque coating films has generally been much more difficult in the past than development of the color separation films; proper development of the opaque coatings has required more "aggressive" developers.
Two problems have been common to prior art developers. First, the developers have had problems of odor, toxicity and flammability, making them difficult to store, handle, use and dispose of. This problem has become particularly serious in recent years as more restrictive environmental and occupational health regulations have come into effect. Second, many prior art developers are not sufficiently selective to cleanly remove unexposed portions of the photosensitive coatings while leaving adequate amounts of the exposed portions of the coatings. This results in films which have "dirty" backgrounds; i.e. where the background portions of the scene which should be clear still retain slight tints of color because the photosensitive color coating was not fully removed. Efforts with prior art developers to produce clean backgrounds result in overly aggressive attack of the exposed portions of the coating, reducing the depth of color or otherwise distorting the color in the areas where the color is to be retained in either the solid or screen areas of the film.
It would therefore be advantageous to have a developer which would be sufficiently aggressive to clean background areas thoroughly and develop opaque coatings while yet being sufficiently selective to produce proper development of color portions of the scenes or copy. In addition, such a developer should be nonflammable, nontoxic and not pose significant odor problems.